While abundant sarcomeric actin isoforms are famous for their essential role in striated muscle contraction, low abundance non-muscle cytoplasmic actin isoforms (?cyto- and ?cyto-actin) are also emerging as important in the maintenance of specialized structures in normal and diseased skeletal muscle. During this project, we generated and characterized muscle-specific mouse lines lacking either ?cyto-actin, or ?cyto-actin, or overexpressing ?cyto-actin to understand their endogenous functions and role(s) in dystrophin-deficient muscular dystrophy. Interestingly, each ?cyto-actin or ?cyto-acin single knockout develops a qualitatively similar phenotype characterized by a progressive myopathy with significant myofiber degeneration/regeneration and muscle weakness. We have shown that 2000-fold muscle-specific overexpression of ?cyto-actin in dystrophin- deficient mdx mice affords significant protection from eccentric contraction-induced force drop. Our new data suggest that eccentric contraction drives a rapidly-reversible, reactive oxygen species (ROS)-mediated inhibition of sarcomeric contractility that may function to protect dystrophic muscles from myofibrillar damage caused by repeated, high force contractions. Finally, we have obtained new data suggesting that ?cyto- and ?cyto-actins collaborate to maintain the functional interaction between mitochondria and sarcoplasmic reticulum. Going forward, we will make use of our unique animal models, isoform-specific reagents and biochemical and physiological methodologies to address fundamental questions about cytoplasmic actins in normal skeletal muscle function and in dystrophin-deficient muscular dystrophy. In aim 1, we will investigate how loss of a key redox buffering protein contributes to eccentric contraction-induced force drop in dystrophic mdx skeletal muscle. In aim 2, we will test the hypothesis that stretch-induced ROS may cause eccentric contraction induced force drop in mdx muscle via reversible oxidative modification of sarcomeric actin or other myofibrillar proteins critical for contractile function. n aim 3, the roles of ?cyto- and ?cyto-actins at the interface between mitochondria and the sarcoplasmic reticulum will be investigated through characterization of mouse lines in which ?cyto-actin and ?cyto-actins have been knocked out in skeletal muscle individually, or in combination. The results of the proposed studies will definitively address the unique and important contributions of cytoplasmic actin isoforms to the function of normal and diseased skeletal muscle.